#include<cstdio>
#include<cstring>
#include<algorithm>

using namespace std;
typedef long long LL;
LL Map[256];
LL inf = (1LL << 63) - 1;
void init(){
	for(char c = '0'; c <= '9'; c++){
		Map[c] = c - '0';
	}
	for(char c = 'a'; c <= 'z'; c++){
		Map[c] = c - 'a' + 10;
	}
}

LL convertNum10(char a[], LL radix, LL t){
	LL ans = 0;
	int len = strlen(a);
	for(int i = 0; i < len; i++){
		ans = ans * radix + Map[a[i]];
		if(ans < 0 || ans > t) return -1;
	}
	return ans;
}

int cmp(char N2[], LL radix, LL t){
	int len = strlen(N2);
	LL num = convertNum10(N2, radix, t);
	if(num < 0) return 1;
	if(t > num) return -1;
	else if(t == num) return 0;
	else return 1;
}

LL binarySearch(char N2[], LL left, LL right, LL t){
	LL mid;
	while(left <= right){
		mid =  (left + right) / 2;
		int flag = cmp(N2, mid, t);
		if(flag == 0) return mid;
		else if(flag == -1) left = mid + 1;
		else right = mid - 1;
	}
	return -1;
}

int findLargestDigit(char N2[]){
	int ans = -1, len = strlen(N2);
	for(int i = 0; i < len; i++){
		if(Map[N2[i]] > ans)
			ans = Map[N2[i]];
	}
	return ans + 1;
}

char N1[20],N2[20], temp[20];
int tag, radix;
int main(){
	init();
	scanf("%s %s %d %d", N1, N2, &tag, &radix);
	if(tag == 2){
		strcpy(temp, N1);
		strcpy(N1, N2);
		strcpy(N2, temp);
	}
	
	LL t = convertNum10(N1, radix, inf);
	LL low = findLargestDigit(N2);
	LL high = max(low, t) + 1;
	LL ans = binarySearch(N2, low, high, t);
	if(ans == -1) printf("Impossible\n");
	else printf("%lld\n", ans);
	return 0;
}
